The present invention relates to analog and digital radio frequency (RF) control techniques, and more particularly to a direct digital systhesizer (DDS)—phase locked loop (PLL) method for frequency sweep that generates and controls accurate and linear wideband frequency sweeps in an analog frequency control section of RF and microwave swept real-time spectrum analyzers.
Generating an accurate linear wideband frequency sweep in swept microwave spectrum analyzers has historically been hindered by a number of nuisance problems. The swept oscillator in these instruments is traditionally a yttrium-iron-garnet (YIG) tuned oscillator (YTO). These oscillators generally have a tuning port for coarse frequency tuning and an FM port for fine frequency tuning. For narrow frequency spans the voltage at the tuning port is held constant and a small ramp signal is applied to the FM tuning port. For wide frequency spans the voltage at the FM tuning port is held constant and a ramp signal is applied to the tuning port. When these oscillators are swept over wide frequency spans, they commonly suffer from non-linearity caused by hysteresis, post-tune drift, frequency offsets, slow response time, intrinsic non-linearity and other problems which substantially hinder the frequency accuracy and linearity of these frequency sweeps. Previously no error correction was used—good design practice was followed to minimize the sweep frequency error.
Another technique used in some spectrum analyzers is to use phase-locked loop (PLL) technology to drive the frequency sweep to provide some sweep error correction. For narrow frequency spans a multi-PLL synthesizer 28 provides one of a plurality of fixed tuning voltages to the YTO and a DSP provides the frequency tuning on the output from an ADC, as shown partially in FIG. 1.
What is desired is a method for frequency sweep in a swept microwave spectrum analyzer that is accurate and linear, correcting for frequency sweep errors over wide frequency spans in a YTO.